1. Field of the Invention
This invention relates to a wheel cylinder adapted for use in a drum brake for vehicles, said wheel cylinder having a mechanism capable of automatic and proper adjustment of an excess clearance formed between the lining and the internal surface of the brake drum due to the wear of the lining.
2. Description of Prior Art
Generally, a drum brake has a pair of shoes provided symmetrical to each other on the inside of the drum, each shoe being anchored at its one end to a backing plate by a pin so that both shoes are pivotable freely relative to each other, the other end of each shoe being attached to an end of the piston in the wheel cylinder. The piston in the wheel cylinder is split into two portions so that both portions can operate independently of each other. Said piston is also so designed that when the brakes are applied, both piston portions are forced out on both sides of the wheel cylinder to let said shoes pivot about the axis of the anchor pins while pressing the lining provided as a friction material on the external surface of each shoe against the interior surface of the brake drum to produce a braking force. When the brakes are taken off, the respective shoes and piston are forced back to their original positions under an elastic force of a return spring disposed between the edge portions of the two symmetrical shoes on the inside of the brake drum. As the wear of said lining adapted as a friction material increases, the piston displacement is accordingly increased to give rise to the problem of brake delay. Accordingly, the wheel cylinder is required to have a mechanism for making automatic adjustment of the clearance between said lining and the inner surface of the brake drum.
The conventional automatic clearance adjusting mechanism is provided on the outside of the wheel cylinder and is designed to directly adjust the respective shoes by utilizing a cam mechanism or a link mechanism independently of the wheel cylinder function. In the brake drum of the type in which an automatic clearance adjusting mechanism is incorporated in the wheel cylinder, there is provided a single-acting wheel cylinder having a single piston, and such type of drum brake is mostly applied to the small-sized vehicles having a brake with only one shoe. Even in the drum brake of the type provided with a double-acting wheel cylinder having two separate pistons and two brake shoes disposed symmetrically to each other on the inside of the drum, the automatic clearance adjusting mechanism is provided in association with only one of the pistons and is designed to adjust only one of the shoes, independently of the movement of the other piston. Further, the automatic clearance adjusting mechanism constructed by using a cam mechanism or a link mechanism is complicated in its own mechanism and has the problem that it is difficult to provide such mechanism in a narrow space in the inside of the brake drum. On the other hand, in the case of the automatic clearance adjusting mechanism of the type in which only one of the pistons is adjusted, there was problem that it is difficult to make proper adjustment of clearance in correspondence to the amount of wear of each shoe lining as the linings of the respective shoes provided symmetrically on the inside of the brake drum have a different degree of wear.